The objective of this study was to conduct the musculoskeletal simulation and the EMG (Electromyogram) measurement for the same trial of the same swimmer performing the breaststroke, and to compare the simulated and measured results, in order to discuss the validity of the musculoskeletal simulator for swimming, which was developed in the previous study. In the experiment, two subject swimmers swam in a circulating tank. The swimming motions were captured by two cameras for the underwater motion and two other cameras for the motion above the water. The EMG were simultaneously measured for eight muscles: triceps brachii, biceps brachii, pectoralis major, latissimus dorsi, deltoid, rectus femoris, biceps femoris, and tibialis anterior. The measured swimming motions were used as the inputs in the simulation. As a result of the comparison between the simulation and experiment, it was found that the performance of the simulation was satisfactory. The simulation could estimate the peak timing as well as the curve shape of the actual muscle activity except for the excessive activation and the biarticular muscle. In addition, the differences in muscle activity due to the subject found in the experiment were sufficiently reproduced by the simulation.